System Reset: Use With Caution

AUTO BRK FAULT at engine start

At engine start of an A320, the BRAKES AUTO BRK FAULT ECAM alert triggered. The flight crew set the A/SKID & N/W STRG switch to OFF and then back to ON (fig.1). This action cleared the alert.

First A/SKID N/WS FAULT during the takeoff roll

The BRAKES A/SKID N/WS FAULT ECAM alert triggered during the takeoff roll prior to the inhibition phase. After the aircraft became airborne, the flight crew cleared the alert by setting the A/SKID & N/W STRG switch to OFF then ON.

Second A/SKID N/WS FAULT during approach

The BRAKES A/SKID N/WS FAULT ECAM alert triggered again on approach. The flight crew set the A/SKID & N/W STRG switch to OFF and ON for a third time and this action cleared the alert.

Main landing gear tires burst at landing

The flight crew noticed unusual braking behavior immediately after touchdown. They applied full reverse thrust and manual braking. The aircraft came to a stop, but the flight crew was unable to begin taxiing. The four main landing gear tires had burst during landing.

Similar faults were observed during the previous flights

There were seven occurrences of BRAKES AUTO BRK FAULT and two occurrences of BRAKES A/SKID N/WS FAULT recorded on this aircraft over the two months prior to the event. System resets and testing the BSCU were the only maintenance actions carried out each time.

Unauthorized resets of the braking and steering functions hid the deteriorating condition of the braking system

There is no reset procedure associated with BRAKES AUTO BRK FAULT and BRAKES A/SKID N/WS FAULT ECAM alerts in the A320 QRH. The use of the A/SKID & N/W STRG switch to reset the braking and steering functions in flight is mentioned in the System Reset table of the QRH, but it is only authorized with the BRAKES SYS 1(2) FAULT and BRAKES BSCU CH 1(2) FAULT ECAM alerts. The unauthorized resets that the flight crew performed during the flight hid the deteriorating condition of the braking system.

Dual tachometer failure

A dual tachometer failure was at the origin of the delay in the braking system activation. The unauthorized in-flight reset prevented the BSCU from detecting the failure of the tachometers and this led to the BSCU considering that 0 kt was the actual wheel speed. After 15 seconds, the braking function was recovered with a default deceleration speed. At the same time, the BSCU was unable to compute the anti-skid order and sent full hydraulic pressure to the brakes and caused the four tires to burst.

A system reset is the action of switching off a system and then switching it back on again with the objective to retrieve normal system behavior or recover a previously lost function. It is different from re-engaging a tripped Circuit Breaker (C/B).

Certain avionic systems, such as the Flight Management System (FMS), have an automatic reset function. The reset action is completely managed by the system that has an automatic failure detection mode. Maintenance or flight crews perform a manual reset by using the cockpit control for the system, a circuit breaker, or a dedicated reset button (also called a reset switch). This article focuses only on these types of manual resets.

For specific systems, such as the flight control system, the maintenance or flight crews can perform a system reset from the cockpit using pushbutton-switches available on the overhead panel (fig.2).

Pulling a system C/B and then pushing it back in will trigger a system reset because this will isolate and then restore the power supply to all parts of the system. It will also cause the software of the system to reload. This is considered as a “hard system reset”.

There are two types of C/Bs: traditional C/Bs and electronic C/Bs. The traditional C/B is manually opened and closed. The electronic C/B, also called Solid State Power Controller (SSPC), is controlled by a remote interface (on A220/A380/A350). Various system C/Bs are located in the cockpit of Airbus A220/A300/A310/A320 aircraft, the avionics bay, the cabin, and the cargo compartments. There are no C/Bs in the cockpit of Airbus A330/A340/A350/A380 aircraft. They are replaced by system reset buttons on the overhead panel.

Pulling a system reset button (fig.3) then pushing it back in the cockpit will only reset the system software part (only available on Airbus A330/A340/A350/A380 aircraft). This is known as a “soft reset” because the system will remain powered.

Past events have highlighted how some system resets can have irreversible consequences. One example is where a system cannot be recovered after an inappropriate system reset in flight. Another example is where a reset of flight control computers is unduly performed. Depending on the system malfunction encountered, this can cause unexpected movements of the flight control surfaces, which may lead to serious consequences if performed in flight.

Avionics systems are interconnected systems, therefore, a system reset of one system can have significant consequences for the other systems that rely on its data. Inappropriate system resets can have unexpected side effects and hide deteriorating conditions of the system. In combination with a failure of another system, the safety of the flight can be impaired. Therefore, it is important that maintenance personnel and flight crews only perform system resets in accordance with the guidance in the relevant procedures, as for the cases described in this article.

The flight crew can perform system resets only in the three cases listed below.

A system reset can be specifically requested in an ECAM/OEB/FCOM/QRH procedure (fig.4).

ECAM procedures do not necessarily request a system reset. In this case, as per the “Airbus Operational Philosophy” section of the FCTM, the PF should call out “STOP ECAM” after they perform the ECAM actions.

Before the review of the status of the aircraft on the STATUS page, if the PF considers that it is necessary to perform a system reset to recover the operation of the affected system, it is the responsibility of the flight crew to first check if such reset is authorized in the System Reset table of the FCOM/QRH.

The System Reset table lists the specific conditions necessary for the reset procedure

The reset procedures are the result of in-depth analysis to define the authorized resets and their associated conditions, such as the maximum number of resets possible, if they are authorized on the ground only, or also in flight. These conditions must be respected.

Read and do the procedure

A reset procedure must be performed in “read and do” mode. The flight crew must not apply the system reset procedure from memory and they must always follow the relevant procedure (ECAM/OEB/QRH/FCOM), or refer to the System Reset table in the QRH/FCOM.

The need for crosscheck

Performing a system reset is an action that can have irreversible consequences. A crosscheck action is necessary before resetting a system when using a guarded cockpit control, a C/B, or a reset button.

Report any reset to maintenance

Any manual system reset that the flight crew does on the ground or in flight must be reported to maintenance and recorded in the aircraft logbook regardless of it being successful (i.e. system recovered) or not. The number of attempted resets should also be specified to help maintenance monitor the resets as an indication of the system condition.

Electrical transients may lead to intermittent system failures

A320 family aircraft systems can be affected by electrical transients during power-up, APU, or engine start, or any electrical transfer.

To manage the side-effects of these transients, depending on the affected system, it is possible to perform system resets from the A320 QRH System Reset table and also system resets that are not specifically listed in the System Reset table.

The affected system is listed in the QRH System Reset table

If the affected system is listed in the QRH System Reset table, the associated reset procedure and conditions must be applied, but only for the corresponding ECAM alerts or system malfunctions listed in the reset table. For other ECAM alerts or malfunctions of the affected system not specifically listed in the System Reset table, a system reset is not authorized.

The affected system is not listed in the QRH System Reset table

If the affected system is not listed in the QRH System Reset table, the system can be reset on the ground only by applying the general guidelines provided in the General part of the System Reset section of the A320 QRH.

Crosscheck and report

As for any reset, a crosscheck must be done when using a guarded cockpit control, a C/B, or a reset button and the reset must be reported in the aircraft technical logbook.

System resets can cause system components to move or operate, which is a risk of serious or fatal injury to people in close proximity to the aircraft. This can also cause damage to the aircraft if in contact with ground equipment. Specific precautions must be taken when performing tasks on or near flight controls, flight control surfaces, around landing gears and landing gear doors, or any other moving parts or components.

Maintenance personnel can perform system resets only in the two cases listed below.

Resets may be required when applying a troubleshooting procedure from the Troubleshooting Manual (TSM) for A300/A310/A320/A330/A340/A380 or the Aircraft Fault Isolation document (AFI) for A350.

Electrical Transients

As already described in Case 3 for flight crew resets, A320 family aircraft operations can be affected by electrical transients at power-on. That is why it is allowable to perform resets on A320 family aircraft that are not part of a dedicated TSM task.

TSM System Reset Table

The A320 TSM contains a list of authorized resets with their associated conditions in the “System Reset Guidelines” task of the ATA 24 chapter. These authorized resets are the same as the authorized resets on the ground in the System Reset table of the A320 QRH.

Fault due to Electrical Transient or Without Previous Record

When it is obvious that a fault is due to electrical transients, or is not present in the fault history, the affected system can be reset under certain conditions. If the affected system is listed in the System Reset table of the ATA 24 TSM task, a system reset is authorized, but only for the ECAM alerts or system malfunctions specifically listed in the table. For other ECAM alerts or system malfunctions not listed in the System Reset table, a reset is not authorized, despite the system itself being listed in the reset table. If the affected system is not listed in the System Reset table, a reset is authorized with no specific restrictions.

Fault not Obviously due to Electrical Transient With Previous Record

If the fault is not obviously due to electrical transients and is present in the fault history (repeated intermittent failure), then the application of the appropriate TSM procedure(s) is required.

Always Record System Resets

An efficient method to record and monitor system resets must be implemented. Any attempted reset must be recorded whether a maintenance reset requested during the fault confirmation part of a TSM/AFI task is successful (intermittent failure, no further action required) or not successful (permanent failure, fault isolation actions to perform).

A Management System for System Resets

Repetitive failures can occur despite maintenance actions, but they may not necessarily reappear over consecutive flights. Operators shall have a dedicated management system for repetitive failures to comply with continued airworthiness regulations.

Repetitive Failure Management

Repetitive resets of the same system could indicate a permanent failure. Appropriate troubleshooting actions must be initiated to mitigate the risk of latent failures on the ground with multiple resets, which could reappear later in flight.

It is important to properly track repetitive occurrences of system malfunctions over several flights. This is usually an indication that the fault condition still exists and deeper troubleshooting is required.